video display unit

Visual display unit

A visual display unit, often called simply a monitor or display, is a piece of electrical equipment which displays images generated from the video output of devices such as computers, without producing a permanent record. A newer monitor typically consists of a TFT LCD, with oldest monitors based around a cathode ray tube (CRT). The monitor comprises the display device, simple circuitry to generate and format a picture from video sent by the signals source, and usually an enclosure. Within the signal source, either as an integral section or a modular component, there is a display adapter to generate video in a format compatible with the monitor.

Screen size

Diagonal size

The size of a display is typically given as the distance between two opposite screen corners. One problem with this method is that it does not distinguish between the aspect ratios of monitors with identical diagonal sizes, in spite of the fact that a shape of a given diagonal span's area decreases as it becomes less square. For example, a 4:3 21" monitor has an area of ~211 square inches, while a 16:9 21" widescreen has an area of only ~188 square inches.

This method of size measurement dates from the early days of CRT television when round picture tubes were in common use, which only had one dimension that described display size. When round tubes were used to display rectangular images, the diagonal measurement of these was equivalent to the round tube's diameter, hence this was used.

Another historically problematic practice is the direct measurement of a monitor's imaging element as its quoted size in publicity and advertising materials. Especially on CRT displays, a substantial portion of the imaging element is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's safe area due to overscan. Seen as deceptive, widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable size.

Imaging technologies

As with television, many different hardware technologies exist for displaying computer-generated output:

Television sets were used by most early personal and home computers, connecting composite video to the television set using a modulator. Resolution and image quality were strongly limited by the display capabilities of television.

Video projectors use CRT, LCD, DLP, LCoS or many other technologies to send light through the air to a projection screen. Front projectors use screens as reflectors to send light back, while rear projectors use screens as diffusers to refract light forward. Rear projectors are often integrated into the same case as their screen.

Cathode ray tube

The CRT or cathode ray tube, is the picture tube of a monitor. The back of the tube has a negatively charged cathode. The electron gun shoots electrons down the tube and onto a charged screen. The screen is coated with a pattern of phosphor dots that glow when struck by the electron stream. Each cluster of three dots, one of each color, is one pixel.

The image on the monitor's screen is usually made up from at least tens of thousands of such tiny dots glowing on command from the computer. The closer together the pixels are, the sharper the image on screen can be. The distance between pixels on a computer monitor screen is called its dot pitch and is measured in millimeters. Most monitors have a dot pitch of 0.28 mm or less.

There are two electromagnets around the collar of the tube which deflect the electron beam. The beam scans across the top of the monitor from left to right, is then blanked and moved back to the left-hand side slightly below the previous trace (on the next scan line), scans across the second line and so on until the bottom right of the screen is reached. The beam is again blanked, and moved back to the top left to start again. This process draws a complete picture, typically 50 to 100 times a second. The number of times in one second that the electron gun redraws the entire image is called the refresh rate and is measured in hertz (cycles per second). It is common in television or very early computer equipment, to use a technique called interlacing, in which all the odd-numbered lines of an image are traced, and then all the even-numbered lines; the circuitry of such an interlaced display need be capable of only half the speed of a non-interlaced display. An interlaced display, particularly at a relatively low refresh rate, can appear to some observers to flicker, and may cause eyestrain.

Display resolution, the number of distinct pixels in each dimension that can be displayed. Maximum resolution is limited by dot pitch.

Dot pitch, describes the distance between pixels of the same color in millimeters. In general, the smaller the dot pitch (e.g. 0.24 mm), the sharper the picture will appear.

Refresh rate, the number of times in a second that a display is illuminated. Maximum refresh rate is limited by response time.

Response time, the amount of time a pixel in a monitor takes to go from active (black) to inactive (white) and back to active (black) again. It is measured in milliseconds (ms). Lower numbers mean faster transitions and therefore fewer visible image artifacts.

Contrast ratio, the contrast ratio is defined as the ratio of the luminosity of the brightest color (white) to that of the darkest color (black) that the monitor is capable of producing.

Aspect ratios, which is the horizontal size compared to the vertical size, e.g. 4:3 is the standard aspect ratio, so that a screen with a width of 1024 pixels will have a height of 768 pixels. A widescreen display can have an aspect ratio of 16:9, which means a display that is 1024 pixels wide will have a height of 576 pixels.

Viewing angle, the ability to be seen from an angle without excessive degradation to the image, measured in degrees horizontally and vertically.

Comparison

CRT

Pros:

Very high contrast ratio (20,000:1 or greater, much higher than many modern LCDs and plasma displays.)

Penetron

See-through for transparent HUDs (although LCDs are also transparent, they are not self-lighting.)

Very high contrast ratios.

Extremely sharp.

Cons:

Color displays are limited to about four tints.

Orders of magnitude more expensive than the other display technologies listed here.

Problems

Dead pixels

A few LCD monitors are produced with "dead pixels". Due to the desire for affordable monitors, most manufacturers sell monitors with dead pixels. Almost all manufacturers have clauses in their warranties which claim monitors with fewer than some number of dead pixels are not broken and will not be replaced. The dead pixels are usually stuck with the green, red, and/or blue sub-pixels either individually always stuck on or off.

Like image persistence, this can sometimes be partially or fully reversed by using the same method listed below, however the chance of success is far lower than with a "stuck" pixel. It can also sometimes be repaired by physically flicking the pixel, however it is always a possibility for someone to use too much force and rupture the weak screen internals doing this.

Stuck pixels

LCD monitors, while lacking phosphor screens and thus immune to phosphor burn-in, have a similar condition known as image persistence, where the pixels of the LCD monitor can "remember" a particular color and become "stuck" and unable to change. Unlike phosphor burn-in, however, image persistence can sometimes be reversed partially or completely. This is accomplished by rapidly displaying varying colors to "wake up" the stuck pixels.

Phosphor burn-in

Phosphor burn-in is localized aging of the phosphor layer of a CRT screen where it has displayed a static bright image for many years. This results in a faint permanent image on the screen, even when powered off. In severe cases it can even be possible to read some of the text, though this only occurs where the displayed text remained the same for years.

This was once a common phenomenon in single purpose business computers. It can still be an issue with CRT displays when used to display the same image for years at a time, but modern computers aren't normally used this way any more, so the problem is not a significant issue. The issue seems to have become exaggerated in popular opinion. The only systems that suffered the defect were ones displaying the same image for years, and with these the presence of burn-in was not a noticeable effect when in use, since it coincided with the displayed image perfectly. It only became a significant issue in 3 situations:

when some heavily used monitors were reused at home,

or re-used for display purposes

in some high security applications (but only those where the high security data displayed did not change for years at a time).

Screen savers were developed as a means to avoid burn-in, but are redundant for CRTs today, despite their popularity.

Phosphor burn-in can be gradually removed on damaged CRT displays by displaying an all white screen with brightness & contrast turned up full. This is a slow procedure, and is usually effective.

Plasma burn-in

Burn-in re-emerged as an issue with early plasma displays, which are more vulnerable to this than CRTs. Screen savers with moving images may be used with these to minimize localized burn. Periodic change of the colour scheme in use also helps.

Glare

Glare is a problem caused by the relationship between lighting and screen, or by using monitors in bright sunlight. LCDs and flat screen CRTs are less prone to reflected glare than conventional curved CRTs, and aperture grille CRTs, which are curved on one axis only, are less prone to it than other CRTs curved on both axes.

If the problem persists despite moving the monitor or adjusting lighting, a filter using a mesh of very fine black wires may be placed on the screen to reduce glare and improve contrast. These filters were popular in the late 1980s. They do also reduce light output.

The above will only work against reflective glare, direct glare (such as sunlight) will completely wash out most monitors' internal lighting, and can only be dealt with by use of a hood or transreflective LCD.

Incomplete spectrum

RGB displays produce most of the visible color spectrum, but not all. This can be a problem where good color matching to non-RGB images is needed. This issue is common to all monitor technologies with 3 color channels.

Display interfaces

Computer terminals

Early CRT-based VDUs (Visual Display Units) such as the DECVT05 without graphics capabilities gained the label glass teletypes, because of the functional similarity to their electromechanical predecessors.

Some historic computers had no modern display, using a teletype, modified electric typewriter, or printer instead.

Composite signal

Early home computers such as the Apple II and the Commodore 64 used a composite signal output to drive a CRT monitor or TV. This resulted in degraded resolution due to compromises in the broadcast TV standards used. This method is still used with video game consoles. The Commodore monitor had S-Video input to improve resolution.

Digital monitors

Early digital monitors are sometimes known as TTLs because the voltages on the red, green, and blue inputs are compatible with TTL logic chips. Later digital monitors support LVDS, or TMDS protocols.

TTL monitors

Monitors used with the MDA, Hercules, CGA, and EGA graphics adapters used in early IBM PC's (Personal Computer) and clones were controlled via TTL logic. Such monitors can usually be identified by a male DB-9 connector used on the video cable. The disadvantage of TTL monitors was the limited number of colors available due to the low number of digital bits used for video signaling.

Modern monochrome monitors use the same 15-pin SVGA connector as standard color monitors. They are capable of displaying 32-bit grayscale at 1024x768 resolution, making them able to interface with modern computers.

TTL Monochrome monitors only made use of five out of the nine pins. One pin was used as a ground, and two pins were used for horizontal/vertical synchronization. The electron gun was controlled by two separate digital signals, a video bit, and an intensity bit to control the brightness of the drawn pixels. Only four shades were possible; black, dim, medium or bright.

CGA monitors used four digital signals to control the three electron guns used in color CRTs, in a signaling method known as RGBI, or RedGreen and Blue, plus Intensity. Each of the three RGB colors can be switched on or off independently. The intensity bit increases the brightness of all guns that are switched on, or if no colors are switched on the intensity bit will switch on all guns at a very low brightness to produce a dark grey. A CGA monitor is only capable of rendering 16 colors. The CGA monitor was not exclusively used by PC based hardware. The Commodore 128 could also utilize CGA monitors. Many CGA monitors were capable of displaying composite video via a separate jack.

EGA monitors used six digital signals to control the three electron guns in a signaling method known as RrGgBb. Unlike CGA, each gun is allocated its own intensity bit. This allowed each of the three primary colors to have four different states (off, soft, medium, and bright) resulting in 64 colors.

Although not supported in the original IBM specification, many vendors of clone graphics adapters have implemented backwards monitor compatibility and auto detection. For example, EGA cards produced by Paradise could operate as an MDA, or CGA adapter if a monochrome or CGA monitor was used in place of an EGA monitor. Many CGA cards were also capable of operating as MDA or Hercules card if a monochrome monitor was used.

Single color screens

Display colors other than white were very popular on monochrome monitors in the 1980s. These colors were more comfortable on the eye. This was particularly an issue at the time due to the lower refresh rates in use at the time causing flicker, plus the use of less comfortable color schemes than used with most of today's software.

Green screens were the most popular color, with orange displays also available. 'Paper white' was also in use, which was a warm white.

Modern technology

Analog monitors

Most modern computer displays can show an infinite number of different colors in the RGB color space by changing red, green, and blue analog video signals in continuously variable intensities. These have been almost exclusively progressive scan since the middle 1980s. While many early plasma and liquid crystal displays have exclusively analog connections, all signals in such monitors pass through a completely digital section prior to display.

While many similar connectors (13W3, BNC, etc…) were used on other platforms, the IBM PC and compatible systems long ago standardized on the VGA connector. All of these connectors deliver nearly flawless high resolution video which vastly outclasses that of a TV.

Digital and analog combination

The first popular external digital monitor connectors, such as DVI-I and the various breakout connectors based on it, included both analog signals compatible with VGA and digital signals compatible with new flat-screen displays in the same connector. This made the connector nearly painless for users of both technologies.

Digital monitors

Newer connectors are being made which have digital only video signals. Many of these, such as HDMI and DisplayPort, also feature integrated audio and data connections. One less popular feature most of these connectors share are DRM encrypted signals, although the HDCP technology responsible for implementing the protection was necessarily rudimentary to meet cost constraints, and was primarily a barrier aimed towards dissuading average consumers from creating exact duplicates without a noticeable loss in image quality.

Configuration and usage

Multiple monitors

More than one monitor can be attached to the same device. Each display can operate in two basic configurations:

The simpler of the two is mirroring (sometimes cloning',) in which at least two displays are showing the same image. It is commonly used for presentations. Hardware with only one video output can be tricked into doing this with an external splitter device, commonly built into many video projectors as a pass through connection.

The more sophisticated of the two, extension allows each monitor to display a different image, so as to form a contiguous area of arbitrary shape. This requires software support and extra hardware, and may be locked out on "low end" products by crippleware.

Primitive software is incapable of recognizing multiple displays, so spanning must be used, in which case a very large virtual display is created, and then pieces are split into multiple video outputs for separate monitors. Hardware with only one video output can be tricked into doing this with an expensive external splitter device, this is most often used for very large composite displays made from many smaller monitors placed edge to edge.

Multiple video sources

Multiple devices can be connected to the same monitor using a video switch, in the case of computers, this usually takes the form of a "Keyboard Video Mouse switch" (KVM) switch, which is designed to switch all of the user interface devices for a workstation between different computers at once.

Virtual displays

Much software and video hardware support the ability to create additional, virtual pieces of desktop, commonly known as workspaces.

Additional features

Power saving

Most modern monitors will switch to a power-saving mode if no video-input signal is received. This allows modern operating systems to power down a monitor after a specified period of inactivity. This also extends the monitor's service life.

Some monitors will also switch themselves off after a time period on standby.

Most modern laptops provide a method of screen dimming after periods of inactivity or when the battery is in use. This extends battery life and reduces wear.

Integrated accessories

Many monitors have other accessories (or connections for them) integrated. This places standard ports within easy reach and eliminates the need for another separate hub, camera, microphone, or set of speakers. Integrated accessories are often of substandard quality.

Directional screen

Narrow viewing angle screens are used in some security conscious applications.

Autopolyscopic screen

A specially designed directional screen which generates 3D images without headgear, distortion or eyestrain.

Touch screen

These monitors use touching of the screen as an input method. Items can be selected or moved with a finger, and finger gestures may be used to convey commands. This does however mean the screen needs frequent cleaning due to image degradation from fingerprints.

Tablet screens

A combination of a monitor with a graphics tablet. Such devices are typically unresponsive to touch, but may offer sensitivity to one or more special tools' pressure, tilt, controls, opposite ends, and multiple tools.